1. Field of the Invention
The present invention relates to polymeric viscosity index improver/dispersant additives for lubricating oils. More particularly, the present invention relates to processes for preparing grafted, and optionally amine-functionalized, hydrocarbon polymers and their use in oil compositions, and includes an improved solution process for grafting unsaturated moieties onto the hydrocarbon polymer by carrying out the graft reaction in a solution medium comprising at least one aromatic ester.
2. Description of Related Art
High molecular weight hydrocarbon polymers, such as hydrogenated copolymers of isoprene and butadiene, copolymers of butadiene and styrene, and ethylene copolymers, particularly ethylene-propylene copolymers are known as viscosity index (V.I.) improving additives for oil compositions, particularly lubricating oil compositions. A substanial body of prior art exists directed towards further reacting these V.I. improvers to form multi-functional V.I. improvers. A multifunctional V.I. improver is a material that improves not only the V.I. properties of the oil, but also imparts dispersancy so as to suspend sludge that may form during the operation or use of the lubricant and to inhibit varnish deposition in engines.
U.S. Pat. No. 3,928,497 discloses a process for preparing a graft-modified ethylene polymer or copolymer which comprises reacting an ethylene polymer or copolymer with a dicarboxylic acid graft-copolymerizable therewith, or its graft-copolymerizable derivative in an alkyl-substituted aromatic hydrocarbon solvent in the presence of a radical initiator, characterized in that the reaction is performed while adding the dicarboxylic acid or its derivative gradually to the solvent in which the starting ethylene polymer or copolymer is present.
U.S. Pat. No. 4,089,794 discloses ethylene copolymers derived from about 2 to 98 wt. % ethylene, and one or more C3 to C28 alpha olefins, e.g. ethylene-propylene, that are solution-grafted under an inert atmosphere and at elevated temperatures with an ethylenically-unsaturated carboxylic acid material in the presence of a high-temperature decomposable free-radical initiator and thereafter reacted with a polyfunctional material reactive with carboxy groups; such as (a) a polyamine, or (b) a polyol, or (c) a hydroxyamine, or mixtures thereof, to form carboxyl-grafted polymeric derivatives, which have good engine sludge and varnish control behavior in fuels and lubricating oils. If the molecular weight is above 10,000, then these polymers are also useful as multifunctional viscosity index improvers.
U.S. Pat. No. 4,517,104 discloses oil soluble viscosity index improving ethylene copolymers, such as copolymers of ethylene and propylene; and ethylene, propylene and diolefin; etc., that are reacted or grafted with ethylenically unsaturated carboxylic acid moieties, preferably maleic anhydride moieties, and reacted with polyamines having two or more primary amine groups and a carboxylic acid component, preferably alkylene polyamine and alkenyl succinic anhydride, such as polyisobutenyl succinic anhydride. Or the grafted ethylene copolymer can be reacted with already formed salts, amides, imides, etc. of said polyamine and acid component, preferably imides of alkylene polyamine and alkenyl succinic anhydride. These reactions are said to permit the incorporation of varnish inhibition and dispersancy into the ethylene copolymer while inhibiting cross-linking with resulting viscosity increase, haze or gelling. The aforesaid grafting reaction may be carried out thermally, or more preferably with a free radical initiator, such as a peroxide, in a mineral lubricating oil, in which case the acid component, preferably also acts to solubilize insoluble compounds formed by side reactions, such as maleic anhydride grafted oil molecules reacted with amine, to thereby inhibit haze formation, particularly when preparing oil concentrates of the V.I.-dispersant additive for later addition to lubricating oils.
U.S. Pat. No. 4,693,838 discloses hydrocarbon polymers, such as ethylene copolymers, that may be reacted with unsaturated nitrogen-containing monomers or unsaturated carboxylic acids in a synthetic hydrocarbon lubricating oil in the presence of a free radical initiator such as a peroxide. The copolymer grafted directly with the nitrogen-containing monomers may be utilized as an additive for oil compositions, particularly lubricating oil compositions as a V.I.-dispersant additive. The polymer reacted with carboxylic acid may be further reacted with amines or amino-alcohols to also form a multifunctional V.I.-dispersant additive.
The disclosures of the foregoing are incorporated herein by reference in their entirety.